1. Field of the Invention
The present invention relates to an optical transmission line system, more particularly to an optical transmission line system using laser light.
Along with the advances made in optical transmission line systems in recent years, the power of the laser light used has been raised to deal with the increasingly long distances of the optical transmission lines.
Therefore, in the case of disconnection or other breakage of the optical fibers used in the optical transmission lines, the laser light emitted from the point of disconnection, etc. may leak out and harm the eyes of the system maintenance personnel. Some measures are required to prevent this.
2. Description of the Related Art
Conventional optical transmission line systems, for example, optical transmission line systems constituted by a first telephone exchange office, a second telephone exchange office, and an optical transmission line connecting these telephone exchange offices, have been constructed as follows. Provision is made of first optical transmission line equipment and second optical transmission line equipment which perform the transmission and reception of downstream transmission signals and upstream transmission signals through a downstream optical transmission line and an upstream optical transmission line. Further, the first and second optical transmission line equipment include first and second optical signal transmission units, first and second optical signal reception units, and first and second light stoppage detecting means provided at the input stages of the first and second optical signal reception units.
Here, for example, if disconnection or other breakage occurs in the middle of the optical transmission line, the stoppage of the received light signal will be detected at the first light stoppage detecting means in the first optical transmission line equipment. When this happens, the operation of the corresponding first optical signal transmission unit will be stopped by the received light signal stoppage detecting signal and the transmission of the optical transmission signal to the downstream optical transmission line will be suspended. By this, the second light stoppage detecting means in the second optical transmission line equipment on the other side will output a received light signal stoppage detecting signal too. This received light signal stoppage detecting signal is applied to the corresponding second optical signal transmission unit to stop the drive of the second optical signal transmission unit. That is, the transmission of the optical transmission signal from the second optical signal transmission unit to the upstream optical transmission line is suspended. Therefore, the laser light leaking from the location of the disconnection or other breakage in the middle of the upstream optical transmission line disappears. Thus, it is possible to prevent damage to the eyes of the systems maintenance personnel who rush to the location of that disconnection or other breakage.
As mentioned above, in the conventional optical transmission line system, when one of the upstream or downstream optical transmission lines (for example, the upstream side) disconnects or breaks, the optical transmission line system stops transmitting and receiving completely despite the fact that there is no disconnection in the other optical transmission line (downstream) and it is normal. The complete cessation of transmission and reception in this way is a problem in that it ends up lowering the working efficiency of the optical transmission line system.